12) 04_blood flow.pdf

REGULATION OF BLOOD FLOW Physiology 2024 Learning Outcomes ØDescribe the factors responsible maintaining the blood flow to the body ØDescribe the structure of microcirculation and name the characteristics of the various vessel types ØDescribe the roles of the smooth muscle cells in the blood vesse...

REGULATION OF BLOOD FLOW Physiology 2024 Learning Outcomes ØDescribe the factors responsible maintaining the blood flow to the body ØDescribe the structure of microcirculation and name the characteristics of the various vessel types ØDescribe the roles of the smooth muscle cells in the blood vessel wall ØName the factors that are responsible for vasoconstriction and vasodilation ØRelate between changes in the microcirculation and the regulation of systemic blood pressure Learning Outcomes ØDescribe peripheral resistance in the circulation ØExplain the relationship between pressure and peripheral resistance ØExplain the relationship between cardiac function, blood vessel filling, blood pressure and blood flow Direction of blood flow • Heart à aorta à arteries à arterioles à capillaries • Capillaries à venules à venes à vena cava à heart • The heart is the pump, • the elastic arteries are pressure reservoirs; • the arterioles direct blood to individual tissues by selectively constricting and dilating (the site of variable resistance); • the capillaries are exchange sites; • the veins hold more than half of the blood in the circulatory system, they are blood reservoirs. Guyton and Hall Textbook of Medical Physiology, 2021 by Elsevier 4 Pressure produced by contraction of the left ventricle is stored in the elastic walls of arteries and slowly released through elastic recoil Human Physiology: An Integrated Approach 6e Pearson Pressure difference Blood pressure is the force that blood exerts against the inner walls of blood vessels. Guyton and Hall Textbook of Medical Physiology, 2021 by Elsevier The pressure is highest in arteries and lowest in veins 6 Blood Vessel Structure The thickness of the smooth muscle– connective tissue layers surrounding the intima varies in different vessels. Interrelationships of Pressure, Flow, and Resistance • Blood flow is determined by two factors: (1) pressure difference of the blood between the two ends of the vessel (2) the impediment to blood flow through the vessel, which is called vascular resistance Guyton and Hall Textbook of Medical Physiology, 2021 by Elsevier Blood flow • the quantity of blood that passes a given point in the circulation in a given period of time • It can be calculated by the following formula, which is called Ohm’s law : • Blood flow is expressed in ml/min or L/min. • Resistance occurs as a result of friction between the flowing blood and the intravascular endothelium all along the inside of the vessel. length (l), the radius (r), dynamic viscosity (η) viscous resistance is proportional to the viscosity of the fluid and the length of the tube but inversely proportional to the fourth power of the radius of the blood vessel. Flow rate should not be confused with velocity of flow • Blood flow means flow rate, the volume of blood that passes a given point in the system per unit of time. • Velocity is a measure of how fast blood flows past a point. • In contrast, flow rate measures how much (volume) blood flows past a point in a given period of time. 11 Regulation of Tissue Blood Flow • Local Control: most tissues have the ability to control their own local blood flow in proportion to their specific metabolic needs. • Humoral Control: control by substances secreted or absorbed into the body fluids, such as hormones (e.g. angiotensin II, vasopressin) and locally produced factors (e.g. kinins, histamine). Local Control of Blood Flow in Response to Tissue Needs • Some of the specific needs of the tissues for blood flow: • Delivery of oxygen to the tissues • Delivery of other nutrients such as glucose, amino acids, and fatty acids • Removal of carbon dioxide from the tissues • Removal of hydrogen ions from the tissues • Maintenance of proper concentrations of ions in the tissues • Transport of various hormones and other substances to the different tissue. Certain organs have special requirements • Blood flow to the skin determines heat loss from the body, helps control body temperature. • Delivery of adequate quantities of blood plasma to the kidneys allows the kidneys to filter and excrete the waste products of the body and to regulate body fluid volumes and electrolytes. Local Control of Blood Flow • Acute control: by rapid changes in local vasodilation or vasoconstriction of the arterioles and precapillary sphincters that occur within seconds to minutes. • Long-term control: slow, controlled changes in flow over a period of days, weeks, or even months. These changes come about as a result of an increase or decrease in the physical sizes and numbers of blood vessels supplying the tissues. Human Physiology: An Integrated Approach 6e Pearson Acute Control of Local Blood Flow • Increases in tissue metabolism increase tissue blood flow • Reduced oxygen availability increases tissue blood flow • Autoregulation of Blood Flow During Changes in Arterial Pressure—Myogenic Mechanisms Vasodilator Theory for Acute Local Blood Flow Regulation • The greater the rate of metabolism or the less the availability of oxygen (or some other nutrients) to a tissue, the greater the rate of formation of vasodilator substances in the tissue cells. • The vasodilator substances diffuse through the tissues to the precapillary sphincters, metarterioles, and arterioles to cause dilation • e.g. adenosine, carbon dioxide, nitric oxide, phosphate compounds, histamine, potassium ions, and hydrogen ions. Guyton and Hall Textbook of Medical Physiology, 2021 by Elsevier The flow of blood through the arteries and arterioles causes shear stress on the endothelial cells because of viscous drag of the blood against the vascular walls. This stress triggers nitric oxide (NO) release Nitric oxide synthase (eNOS) enzyme in endothelial cells synthesizes NO from arginine and oxygen. NO activates soluble guanylate cyclases, resulting in conversion of cyclic guanosine triphosphate (cGTP) to cyclic guanosine monophosphate (cGMP), which ultimately causes the blood vessels to relax. Guyton and Hall Textbook of Medical Physiology, 2021 by Elsevier • The precapillary sphincters and metarterioles open and close cyclically several times per minute, with the duration of the open phases being proportional to the metabolic needs of the tissues for oxygen. • The cyclical opening and closing is called vasomotion . Myogenic theory • Sudden stretch of small blood vessels causes the smooth muscle of the vessel wall to contract. • high arterial pressure stretches the vessel, reactive vascular constriction results, which reduces blood flow nearly back to normal. • at low pressures, the degree of stretch of the vessel is less, so the smooth muscle relaxes, reducing vascular resistance and helping to return flow toward normal. • Myogenic contraction is initiated by stretch-induced vascular depolarization, which then rapidly increases calcium ion entry from the extracellular fluid into the cells, causing them to contract. The sympathetic nervous system innervate smooth muscle in blood vessel walls • Contraction of smooth muscle à reduction in the diameter of the vessel (vasoconstriction). • If vasomotor impulses are inhibited, the muscle fibers relax, and the diameter of the vessel increases (vasodilation). • Changes in the diameters of arteries and arterioles greatly influence blood flow and blood pressure. • Changes in the diameter of veins affect the amount of blood returning to the heart. 27 Human Physiology: An Integrated Approach 6e Pearson b2-receptor subtype has a higher affinity for epinephrine 29 Peripheral resistance Changes in arteriole diameters regulate peripheral resistance. Blood vessels with smaller diameters offer a greater resistance to blood flow, factors that cause arteriole vasoconstriction increase peripheral resistance, which raises blood pressure. Peripheral resistance is under tonic sympathetic control. Increased sympathetic activity à vasoconstriction Increased sympathetic activity constricts veins à increase venous return 30 Cardiac output and venous return each can be examined separately as a function of right atrial pressure Vascular function curve • Left ventricular end-diastolic volume depends on venous return, which also determines right atrial pressure. • Venous return back to the heart is driven by a pressure gradient. • Thus, as right atrial pressure increases, this pressure gradient decreases, and venous return also decreases. Cardiac and vascular function curves *Mean systemic pressure is the value for the right atrial pressure at which venous return is zero and right atrial pressure is at its highest value 33 Cardiac and vascular function curves Right atrial pressure is related to venous return, end-diastolic volume, and end-diastolic fiber length: As venous return increases, right atrial pressure increases, and end-diastolic volume and end-diastolic fiber length increase. Increases in end-diastolic fiber length produce increases in cardiac output (Frank-Starling mechanism). When right atrial pressure reaches a value of approx. 4 mm Hg, cardiac output can no longer keep up (approx. 9 L/min). The cardiac function curve is cardiac output as a function of right atrial pressure. The vascular function curve is venous return as a function of right atrial pressure. The curves intersect at the point (filled circle) where cardiac output and venous return are equal. 34 Combining these curves provides a useful tool for predicting the changes in cardiac output that will occur when various cardiovascular parameters are altered 35 The point at which the two curves intersect is the unique operating or equilibrium point of the system in the steady state. 36